OBJECTIVE: This study was undertaken to analyze the relationship between the fetal electrocardiogram and arterial pH and base excess at delivery. STUDY DESIGN: In the labor wards of three teaching hospitals a retrospective observational study was conducted on fetal monitor recordings of 679 women for whom there was an indication for continuous fetal monitoring during labor. These women had been recruited as part of either observational studies or a prospective randomized trial related to the Nottingham fetal electrocardiographic project. Fetal heart and uterine contraction data were obtained with the Nottingham fetal electrocardiographic analyzer. Morphologic and time interval analyses of the fetal electrocardiogram were performed. Evaluation was carried out for the last half hour before delivery. Main outcome measures were umbilical arterial pH and base excess at delivery. RESULTS: The study demonstrated a relationship between time interval analysis of the fetal electrocardiogram and a low umbilical arterial pH and base excess at delivery. Analysis of the morphologic characteristics of the fetal electrocardiogram (ST segment and T-wave height) showed no significant relationship. CONCLUSIONS: Time interval analysis of the fetal electrocardiogram during labor is related to relative acidemia at delivery.